The present invention relates to a centrifugal separation device and method of separating solids or swarf from contaminated effluent such as oil base coolants and other liquids which are used in a variety of grinding and machining applications in the glass, ceramic and metal-forming industries. In a centrifugal separator, the separation of swarf from the liquid is commonly accomplished by pumping the contaminated coolant or liquid into a high speed rotating chamber or bowl. The centrifugal gravitational forces created by the high speed rotation of the chamber cause the contaminated fluid to conform to the interior outside vertical surface of the rotating chamber. Since the chamber is rotating at a high speed, the solid material is forced to adhere to the side of the bowl or chamber while the cleansed coolant or liquid exits through openings commonly located at the bottom of the bowl to be drained away through an outlet pipe. Such centrifugal separators provide a rugged, simple and cost effective way to maintain the clean fluid necessary for consistent product quality and long tool life in the glass, ceramic and metal-forming industries. Centrifugal separators are commonly capable of recovering up to 95% of reusable fluid and provide a discharge of solids that is relatively moisture free.
It is axiomatic that the efficiency of a centrifugal separator decreases as increasing amounts of solid material build up on the sides of the separator. Therefore, the separator must be periodically stopped to remove the solid cake material or swarf. Separators have been provided which capture the solids on a removable liner which can be easily removed, cleansed and replaced by an operator. Automatic cleansing systems have been provided wherein scraper blades mounted inside the rotating bowl are activated to automatically scrape or plow the solids from the side of the bowl and expel them into a sludge container mounted below the unit. Commonly, the cleansing cycle is provided at timed intervals which are usually determined by an operator based upon experience. Problems have been encountered with timed cleansing operations due to the potential for variations to occur in the amounts of solids which might flow through the separator between cleansing intervals. If varying amounts of solids build up on the interior of the separator, it is difficult to ensure that the separator will not become overloaded or inefficient. An excess of removable solids between the timed cleansing intervals can result in the following deficiencies in the removal system: inefficient solid separation; overloading or premature failure of the bowl drive motor and mechanics; and overloading or premature failure of the scraper drive motor and mechanics.
An example of such commonly encountered difficulties is presented by viewing the differences between metal and glass particles. Glass particles, having a specific gravity of approximately 2.2 to 2.6, are minimal in weight as compared to metal particles which have a specific gravity of 8 to 9. Thus, the load build-up of glass on the inner wall of the rotating bowl is slower than the load build-up of metal, when the solid to liquid ratios are the same. However, glass particles, upon becoming dehydrated, have a tendency to adhere to one another and to the inner wall of the rotating bowl making the glass difficult to remove in the scraping process. As a result, even though glass has a lower specific gravity, it is often the case that the load sensing and scraping cycle for glass must be done at more frequent intervals.
Attempts to provide for and ensure the efficient operation of such centrifuge separator units are evidenced by the following patents. U.S. Pat. No. 4,522,620 discloses a method and apparatus for measuring the quantity of solid material in a centrifuge cylinder using a mechanical vibration system which senses the quantity of solids. A supplemental mass is attached to the cylinder to produce proportional vibrations by increasing or decreasing the speed of the cylinder drive shaft. A sensor measures the frequency of the vibrations and sends signals to a controller which calculates the quantity of the load of solid material from the vibrational frequencies. U.S. Pat. No. 4,773,992 discloses a centrifuge system for removing impurities from metal working coolant having a timing means to signal the discharge of the solids. U.S. Pat. No. 4,952,127 discloses a method and apparatus for separating high molecular weight substances from fluid culture medium wherein an optical sensor is used to monitor the solid content of the centrifuge. U.S. Pat. No. 5,095,451 discloses a centrifuge particle size analyzer which creates measurements by means of a radiation source such as an x-ray as a function of intensity, time and radial position to determine the distribution and particle size of solids within the centrifuge. U.S. Pat. No. 5,253,529 discloses a system for measuring constituents of a centrifuged medium using a mechanical signal such as a sound wave.
The prior art thus described teaches a variety of complex methods and apparatus for sensing the quantity and particle size of solids retained in a centrifuge. Such designs, for many varied reasons, are often undesirable for centrifugal separators intended for heavy, rugged field use. Thus, the present invention meets a demand for a rugged, low maintenance centrifuge having a simple, cost effective way to maintain the clean fluid necessary to achieve consistent product quality and long tool life for glass, ceramic and metal-forming applications.